As seen from FIG. 1, conventionally, this kind of contour emphasizing circuit comprises a horizontal contour detecting stage 29, a vertical contour detecting stage 30, an adding circuit 31 for giving the sum of the output of the horizontal contour detecting stage 29 and the output of the vertical contour detecting stage 30, and another adding circuit 32 for adding the contour component signal outputted from the adding circuit 31 to the video signal.
Said horizontal contour detecting stage 29 comprises a contour detecting filter 19a directly connected to a video signal input terminal 10, a contour detecting filter 19b connected to the same through 1 set of 1-dot delay circuit 11, a contour detecting filter 19c connected to the same through 2 sets of 1-dot delay circuit 11, an adding circuit 20 for giving the sum of the outputs from the contour detecting filters 19a, 19b and 19c, a coefficient multiplying circuit 21 for multiplying a predetermined coefficient K1, and a 1-line delay circuit 34 connected to the coefficient multiplying circuit 21. More particularly, the horizontal contour components are detected from the objective pixel and the pixel preceding by 1 dot and the pixel immediately following the objective pixel. The delay circuit 34 is provided for synchronizing the timing of the output of the vertical contour detecting stage 30 with it.
The vertical contour detecting stage 30 comprises the contour detecting filter 19a connected to the video signal input terminal 10 through 1 set of 1-dot delay circuit 11, the contour detecting filter 19b connected to the same through 1 set of 1-dot delay circuit and 1 set of 1-line delay circuit, the contour detecting filter 19c connected to the same through 1 set of 1-dot delay circuit 11 and 2 sets of 1-line delay circuit 12, the adding circuit 20 for outputting the sum of the outputs of the contour filters 19a, 19b and 19c, and the coefficient multiplying circuit for multiplying a predetermined coefficient K2. In other words, the vertical contour components are detected from the objective pixel and the pixel preceding the objective pixel by 1 dot and the pixel immediately following the objective pixel.
Now, let's assume that the digital video signals respectively representing the contours in the horizontal direction and the vertical direction are inputted to a conventional circuit as is described above.
This case refers to one where the differences in luminance occur consecutively, and thus the differences in the luminance not occurring consecutively are not considered to be contours. In the example shown in FIG. 2(a), the portions where luminance [8] and luminance [4] occur consecutively are recognized as contours.
In such a case, in processing by the horizontal contour detecting stage 29, when the coefficients are given as −¼, 2/4 and ¼ respectively, the outputs of the contour detecting filters 19a, 19b and 19c become    8×(−¼), 8×( 2/4) and 8×(−¼)=−2, +4 and −2 respectively    when the pixel at the third row and the first column in FIG. 2(a) is picked out as an objective pixel, and the output of the adding circuit 20 becomes 0. Similarly, when the pixel at the intersection of the third row and the second column is picked out as an objective pixel X2, they become    8×(−¼), 8×( 2/4) and 4×(−¼)=−2, +2 and −1 respectively,    and the output of the adding circuit 20 becomes +1;    when the pixel at the intersection of the third row and the third column is picked out as an objective pixel X3, they become    8×(−¼), 4×( 2/4) and 4×(−¼)=−2, +2 and −1 respectively,    and the output of the adding circuit 20 becomes −1;    when the pixel at the intersection of the third row and the fourth column is picked out as an objective pixel X4, they become    4×(−¼), 4×( 2/4) and 4×(−¼)=−1, +2 and −1 respectively,    and the output of the adding circuit 20 becomes 0.
In the example shown in FIG. 2(a), only these four cases are available. When the coefficient in the coefficient multiplying circuit 21 is given as K1=1, the output of said circuit is as shown in FIG. 2(b).
Since only 4 cases, namely, Y1, Y2, Y3 and Y4 are available even when processing by the vertical contour detecting stage 30, similarly to the cases described above, when the coefficient of the coefficient multiplying circuit 21 is given as K2=1, the output is as shown in FIG. 2(c).
When these values are added by the adding circuit 31 and then added to the original video signals by the adding circuit 32, the luminance of the contour having the luminance [8] becomes 8+1=9, while the luminance of the contour having the luminance [4] becomes 4−1=3, thereby emphasizing the contour at the video signal output terminal 23 as shown in FIG. 2(c).
Next, let's assume a case where the digital video signals representing rightward-rising contours with the luminance [8] and the luminance [4] are inputted to the video signal input terminal 10 of the a circuit having the conventional composition.
In such a case, when processing by the horizontal contour detecting stage 29 and when the pixel at the intersection of the third row and the third column is picked out as the objective pixel X1, the outputs of the contour detecting filters 19a, 19b and 19c become    8×(−¼), 8×( 2/4) and 4×(−¼)=−2, +4 and −1    and the output of the adding circuit 20 becomes +1,    whereas when the pixel at the intersection of the third row and the fourth column is picked out as the objective pixel X2, they become    8×(−¼), 4×( 2/4) and 4×(−¼)=−2, +2 and −1    and the output of the adding circuit 20 becomes −1.
The same results will be obtained when the processing is made by using the vertical contour detecting stage 30.
When the pixel at the intersection of the third row and third column is picked out as the objective pixel Y1, they become    8×(−¼), 8×( 2/4) and 4×(−¼)=−2, +4 and −1    and the output of the adding circuit 20 become +1,    while when the pixel at the intersection of the third row and the fourth column is picked out as the object pixel Y2, they become    8×(−¼), 4×( 2/4) and 4×(−¼)=−2, +4 and −1    and the output of the adding circuit 20 becomes −1.
These values are added by the adding circuit 31 and are further added to the original video signals by the adding circuit 32 to emphasize the contours as shown in FIG. 3(b).
As shown in FIG. 2(a), when the digital signals representing the horizontal contours and the vertical contours respectively are inputted, at the point where the horizontal contour and the vertical contour intersect each other, both the horizontal contour and vertical contour respectively having the high luminance [8] will not be emphasized as indicated by the circle in FIG. 2(d), thereby maintaining the luminance [8], while the horizontal contour and the vertical contour of the contour respectively having lower luminance [4] will be emphasized to the level of 4−1−1=2 to further emphasize the contour, since both the horizontal contour emphasis and vertical contour emphasis apply in this case. Thus, it has a problem that any one luminance differing in level from a series of uniform luminances will become too conspicuous.
Further, in the case where the digital video signal representing the contour lying at 45° as shown in FIG. 3(a) is inputted, the signal is processed assuming that the points where the horizontal contour and the vertical contour intersect each other occur consecutively, so that, as shown in FIG. 3(b) by circles, the contours having the high luminance [8] are emphasized to a level of 8+1+1=10 because of the combined effects of the horizontal contour emphasis and vertical contour emphasis, and this also occurs in the cases of the contours having the lower luminance [4], since the luminances of the contours are emphasized to a level of 4−1−1=2 because of the combined effects of emphasis on the horizontal contour and the emphasis on the vertical contour. This has been a problem of the prior art, that is, for instance, the luminance [8] of the inclined contour is overemphasized to [10] while the luminance [4] is to [2] to make the emphasized contours too conspicuous.
An object of the present invention is to provide a contour emphasizing method and circuit designed for being capable of detecting the directions of the contours so that the signals representing the contours at the points where the horizontal contour and the vertical contour intersect and the inclined contours can be processed more naturally than by the conventional method.